JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.microsoft.com/ajax/jquery.validate/1.7/jquery.validate.min.js"></script>
<form>
    <select name="mySelect" multiple="multiple">
        <option>1</option>
        <option>2</option>
        <option>3</option>
        <option>4</option>
    </select>
    <button type="submit">Submit</button>
</form>

JavaScript

jQuery.validator.addMethod("maxtwo", function(value, element) {
  return $(element).find(":selected").length <= 2;
},"Max two options.");

$("form").validate({ rules : { mySelect: { maxtwo: true } } });